During the normal operation of a computer, integrated circuit devices generate significant amounts of heat. This heat must be continuously removed, or the integrated circuit device may overheat, resulting in damage to the device and/or a reduction in operating performance. Cooling devices, such as heat sinks, have been used in conjunction with integrated circuit devices in order to avoid such overheating. Generally, a passive heat sink in combination with a system fan has provided a relatively cost-effective cooling solution. In recent years, however, the power of integrated circuit devices such as microprocessors has increased exponentially, resulting in a significant increase in the amount of heat generated by these devices, thereby necessitating a more efficient cooling solution.
Heat sinks operate by conducting heat from the processor to the heat sink and then radiating it into the air. The better the transfer of heat between the two surfaces (the processor and the heat sink metal) the better the cooling. Some processors come with heat sinks glued to them directly, or are interfaced through a thin and soft layer of thermal grease, ensuring a good transfer of heat between the processor and the heat sink.
FIG. 1 shows a conventional heat sink assembly 100 attached to a microprocessor. This conventional heat sink 100 would most commonly be used in conjunction with a separate fan unit. Directional airflow provided by the fan ensures efficient heat transfer from the fins (102) of the heat sink 100 to the ambient air. Thermal grease 104 is used as a conductive substance to transfer heat from the processor die (chip) 110 to the heat spreader 106 and onto the heat sink fins 102. The thermal grease 104 helps bond the two surfaces and transfer the heat more effectively. The processor die (chip) 110 is secured to the ceramic base 112 using a die attach method incorporating solder balls 120.
An enhancement on the above structure is sometimes achieved by adding a fan (not shown) that blows cooling air at the chip package. Such a conventional fan heat sink assembly suffers from two types of inefficiencies. First, the airflow generated by the fan blades diffuses into the open space and only a portion of the airflow impinges on the fins 102 of the heat sink 100. Even with proper shrouding, added to direct the airflow over the fins of a heat sink, the fins located downstream from the airflow see less vigorous thermal interaction with the impinging air due to the deceleration of the airflow and the build up of a boundary layer. Second, as processing power continues to increase, the combined mass/volume of the fan-heat sink system needs to grow in scale accordingly, thus making the fan-heat sink configuration less competitive in a size-driven market.
By integrating a fan with a passive heat sink module, a heat sink configuration of smaller size and increased thermal efficiency (i.e., reduced thermal resistance or increased thermal conductivity) is obtained. Fan heat sinks (a heat sink with integrated fan) have been used to address the greater cooling needs of today's microprocessors, and in some cases they provide the only viable solution. An example of this would be the cooling of embedded personal computers (PCs) in hazardous industrial locations where particles or corrosive fumes occupy the same environment as the PC. Sealing the chassis is the best way to ensure long life. Fan heat sinks can cool sensitive chips in this sealed environment, offering a solution where traditional methods fail. Some low-end personal computers (PCs) use fan heat sinks in place of a system fan. This solution lowers the overall cost and provides a quieter system. Since office noise level requirements are stricter in Europe, many PCs made for the European market use fan heat sinks specifically to lower the noise level
An integrated fan improves the performance of any heat sink by about 50 to 100%. The directional airflow provided by the fan ensures more efficient heat transfer from the fins 102 to the ambient air. When fan heat sinks are engineered into the system they can provide many years of uninterrupted service. —From “Fan Heat Sinks for High Performance Socket ICs” by Chris Chapman, Vivek Mahnsingh and Prabhu Sathyamurthy, 2002. However, as functionality and processor speed increase, conventional fan heat sinks will no longer be able to supply the total cooling for systems, requiring alternative cooling strategies.
The effectiveness or heat transfer capability of a heat sink is a function of its surface area, the temperature difference between the heat sink and the fluid (air) moving past the heat sink, and a heat transfer coefficient hc. The heat transfer coefficient hc in turn depends upon such factors as the geometry of the fluid flow and its velocity past the heat sink surfaces. The equivalent thermal resistance of an airflow-based cooling system is governed by three factors:                1. heat resistance of the heat sink (this is determined by the material and the effective distance to the heat source);        2. effective surface area exposed to convection airflow; and        3. rate of airflow over heat transfer area.        
Current heat sinks address the cooling needs of today's microprocessors, but in order to address the increasingly complex and expensive cooling needs of the next generation microprocessors, there is a need for a thermal cooling system which can overcome the shortcomings of the prior art by providing superior cooling efficiency in a compact form factor.